The present invention relates to a method of producing pattern plates for etching. More particularly, the present invention relates to a method of producing a pair of obverse and reverse working pattern plates for use in the manufacture of etched products such as shadow masks for color picture tubes, lead frames for semiconductor devices, etc., and a pair of obverse and reverse master pattern plates used to produce such working pattern plates. The present invention further relates to a photolithographic apparatus which may be employed to carry out the above-described pattern plate producing method.
Etched products such as shadow masks are generally produced by exposing a metal plate having a resist coated over the obverse and reverse sides thereof through a pair of obverse and reverse working pattern plates, each formed with a predetermined etching pattern, the working pattern plates being in close contact with the two sides of the metal plate, and developing the resist to form resist patterns on the obverse and reverse sides of the metal plate, and then etching the two sides of the plate formed with the respective resist patterns.
Working pattern plates which are used in the actual etching process are produced by use of original pattern plates, which are known as master pattern plates. More specifically, a pair of obverse and reverse master pattern plates are first produced, and then working pattern plates are produced by use of the respective master pattern plates. For experimental etching or small-scale production, a photosensitive glass plate having the required pattern formed thereon by a photolithographic machine may be used directly as a working pattern plate. In the case of mass production, for example, it is common practice to produce working pattern plates by use of master pattern plates and use the working pattern plates in etching process carried out along a production line.
In general, of the two working pattern plates, one which has a hole pattern with a larger diameter is called "obverse working pattern plate", and the other "reverse working pattern plate".
In the production of a pair of obverse and reverse pattern plates, it has been conventional practice to divide all the patterns required for etching into a plurality of discrete patterns, i.e., a pattern of marks used in the alignment of a pair of obverse and reverse pattern plates (these alignment marks will hereinafter be referred to as "register marks"), a frame pattern, a pattern of holes, etc., and to form these patterns on a single photosensitive plate in the form of a synthetic pattern by partial exposure or multiple exposure, as disclosed, for example, in Japanese Patent Post-Examination Publication Nos. 63-19860 and 63-19861 (1988).
However, the above-described conventional method requires a great deal of time and labor to carry out the operation since it is necessary to effect multiple exposure with a plurality of discrete pattern plates being successively brought into contact with a single photosensitive plate.
Because in the etching process along a production line a pair of obverse and reverse working pattern plates are brought into close contact with both sides of a steel plate which are coated with a resist, the two working pattern plates are formed with register mark patterns to align the positions of the obverse and reverse etching patterns relative to each other. Hitherto, since the obverse and reverse master pattern plates are formed by multiple exposure, when each master pattern plate is formed, the relative position of the patterns to be formed into a synthetic pattern may change due to a manual operation, which gives rise to a problem in terms of quality, that is, nonuniformity in the quality of the products.
In addition, no matter which of various known photolithographic apparatuses is used to write etching patterns, the resulting patterns involve various distortions, more or less, i.e., a distortion in orthogonality, that is, an error in which an angle which should be a right angle is deviated therefrom, or a distortion in length, that is, an error in which a length becomes longer or shorter than the designed one. Thus, any etching pattern that is written on an original pattern plate has some distortions; under certain circumstances, distortions may be present everywhere on a single original plate. The tendency of distortions is the same for all patterns that are written by the same photolithographic apparatus under the same conditions. Accordingly, if the same photolithographic apparatus is used to produce a pair of obverse and reverse original pattern plates and etching is effected by use of working pattern plates that are produced on the basis of these original plates, distortions that are produced by this photolithographic apparatus will double.